For decades, the prevailing paleoclimatological consensus attributed the sudden onset of the Younger Dryas—a severe, 1,200-year cold interval beginning approximately 12,900 years ago—exclusively to the catastrophic outburst of glacial Lake Agassiz, which routed freshwater into the North Atlantic and shut down the Atlantic Meridional Overturning Circulation (AMOC). However, recent high-resolution analysis of lacustrine sediment cores from western Ireland has complicated this singular attribution. Researchers discovered that local shifts in oceanic surface temperatures and terrestrial vegetation assemblages preceded the widespread slowdown of the AMOC by nearly two centuries. Specifically, microfossil analysis of Chironomidae (non-biting midges) reveals a sharp 3°C drop in summer surface air temperatures accompanied by an abrupt decline in Betula (birch) pollen density prior to any detectable disruption in marine isotope proxy signals associated with thermohaline circulation. While these findings do not completely refute the role of meltwater discharge in sustaining the protracted cooling phase of the Younger Dryas, they demonstrate that atmospheric circulation shifts initiated local thermal deterioration well before the catastrophic hydrological discharge altered basin-wide oceanic circulation.
Based on the passage, which of the following statements regarding the onset of the Younger Dryas is directly supported by the text?
Select all that apply.
- The initial drop in local summer surface air temperatures in western Ireland occurred before any measurable disruption in thermohaline circulation proxy signals.Answer
- BThe findings from western Ireland completely disprove the hypothesis that meltwater discharge from Lake Agassiz contributed to Younger Dryas cooling.
- An abrupt reduction in Betula pollen density was documented prior to the widespread slowdown of the AMOC.Answer